The invention relates to controlling a rotation speed of a motor of a speed-controllable hoist drive.
When a load is lifted from the ground, both the load and the structure carrying the load are subjected to vertical vibrations. The vertical vibration is mainly caused by an impact load which is generated when the load is quickly lifted from the ground at a high lifting speed.
The impact load may be reduced by keeping the lifting speed low when removing the load from the ground. An experienced hoist operator may apply this method manually by reducing the lifting speed at a point of time when the load comes off the ground.
It is known to equip a hoist drive with a hoist controller arranged to detect the tightening of a cable and the load becoming airborne by monitoring a change in the cable force relative to time, i.e. the time derivative of the cable force. When the time derivative of the cable force becomes too high, the lifting speed is reduced. When the time derivative of the cable force becomes sufficiently low, the lifting speed is raised back to its original value. Such a controller enables quite good results to be achieved in connection with two-speed hoist drives.
A problem with the prevention of impact load based on monitoring the time derivative is that the method is not very well suited to speed-controllable hoist drives wherein the lifting speed may be anything between minimum and maximum speeds.